DBC technology has been used in the electronics industry for bonding copper to alumina Al.sub.2 O.sub.3 (cf. J. F. Burgess et al., "Hybrid packages by the direct bond copper process", Solid State Science and Technology, May 1975, pp. 40).
That technique is highly advantageous since it makes it possible to bond copper to the oxide without using an intermediate layer (e.g. solder) which in general reduces heat transfer. It is based on the formation of a copper-oxygen eutectic at 1065.degree. C., as indicated in the following two articles:
J. F. Dickson, "Direct bond copper technology: materials, methods, applications", Int. J. Hybrid Microelectronics, 5 (2), 103-109 (1982); and PA1 J. E. Holowczak, V. A. Greenhut, O. J. Shanefield, "Effect of alumna composition on interfacial chemistry and strength of direct bonded copper-alumina", Ceram. Eng. Sci. Proc. 10 [9-10], pp. 1283-1294 (1989). PA1 P. Kluge-Weiss, J. Gobrecht, "Direct bonded copper metallization of AlN substrates for power hybrids", Mat. Res. Soc. Symp. Proc. 40 (1985); and PA1 W. L. Chiang, V. A. Greenhut, D. J. Shanefield, L. Sahati, R. L. Moore, "Effect of substrate and pretreatment on copper to AlN direct bonds", Ceram. Eng. Sci. Proc. 12 [9-10], pp. 2105-2114 (1991).
The copper is placed in contact with the ceramic and they are heated together to a temperature lying in the range 1065.degree. C. (eutectic temperature) to 1083.degree. C. (copper melting temperature) in an inert atmosphere. The oxygen required for forming the eutectic may be injected directly into the atmosphere or else it may be provided by previously oxidizing the copper. At the treatment temperature, the resulting liquid wets both the ceramic and the copper, thus forming a bond between the two antagonists, after cooling. A copper aluminate is also formed, reinforcing the adherence of the copper.
Although that technique has been applied until now essentially to alumina, it is known that it is also capable of bonding copper to silica, to beryllium oxide (BeO), to sapphire, or to certain compounds of the spinel type, all of which materials are oxides.
The new high thermal conductivity ceramics for power electronics, such a aluminum nitrate (AlN) for example, can be bonded to copper by that technique only if they are previously surface oxidized. Non-oxides (such as AlN) are not wetted by copper or by the CuO eutectic, and it is not possible to form a copper aluminate.
The surface oxidizing of aluminum nitride with aluminate is a major parameter that conditions the quality with which copper adheres to aluminum nitride, as mentioned in European patent application EP-A-170 012 and in the following two articles:
In particular, too great a thickness of the resulting alumina layer gives rise to stresses between AlN and Al.sub.2 O.sub.3, thereby reducing the quality of the subsequent Cu--AlN bond. The thickness of the oxide layer must be as small as possible, but must nevertheless remain sufficiently large since this layer is completely dissolved in the Cu--O eutectic if less than 0.1 .mu.m thick.
Until now, the nitride has been oxidized in an oven under air or under oxygen at temperatures greater than 1100.degree. C. for periods of time exceeding 1 hour. The method is lengthy and it does not enable oxidizing to be performed selectively at precise locations on the surface of the aluminum nitride. In addition, the method cannot be applied to thin layers of aluminum nitride or to thick layers that are supported by oxidizable substrates.